Pour vinaigrette into a bowl. Dip steaks into vinaigrette and set aside on a plate; set aside vinaigrette. Warm a large nonstick skillet over medium-high heat. Add 1 Tbsp. of oil that has risen to top of vinaigrette bottle. (If vinaigrette hasn't separated, mist skillet with cooking spray. Add onions; cook until slightly softened, 5 to 6 minutes. Push to side of pan.
Season steaks with salt and pepper. Add 2 steaks to pan. Cook, turning once , until nicely browned on both sides, 5-6 minutes for medium; stir onions occasionally. Remove steaks to serving plates and tent with aluminum foil to keep warm. Repeat with remaining 2 steaks.
Whisk cornstarch and 2 cup water into reserved vinaigrette; add to skillet. Cook, stirring constantly, until mixture thickens. Stir onions into sauce. Taste, add salt if necessary and spoon sauce over steaks.